1. Field of the Invention
The present invention relates to a developing apparatus employed in an image forming device of an electrophotographic copying machine or the like.
2. Description of the Prior Art
In a developing apparatus of this type, a developer includes toner particles and carrier particles and is housed in a casing. The thickness or height of the developer supplied onto a developing sleeve from a developer-stirring and supplying device is regulated by means of a height-regulating member. In order to return an excessive portion of developer removed from the sleeve by height-regulating member to the developer-stirring and supplying device, there is provided a partition plate constructed so that an upstream portion thereof in the direction of returning movement of the developer is horizontal and a downstream portion thereof connected with the upstream portion is inclined diagonally downwardly, e.g. as disclosed in U.S. Pat. No. 4,666,283.
However, in a developing apparatus of this type, the toner concentration of the developer to be returned is detected by means of a toner-concentration sensor, and upon detection that such concentration has become less than an appointed set value, new toner particles are supplied. Accordingly, it is necessary that the detection of the toner concentration of the developer returned over such partition plate be of high accuracy. Thus, a detecting surface of the toner-concentration sensor is provided on an upper wall surface of the case so as to be parallel to the horizontal upstream portion of the partition plate. However, the above described conventional developing apparatus has the disadvantage that it thereby is necessary that the height of the case be increased, and thus the developing apparatus becomes large-sized.
In order to solve such problem, a construction as shown in, for example, FIG. 15 was considered. That is to say, referring to FIG. 15, reference numeral 51 designates a photoreceptor rotating in the direction of an arrow R with an axis of rotation 51a as a center. Reference numeral 52 designates a case for housing therein a developer comprising toner particles and carrier particles. Case 52 is provided with an open portion 53 facing photoreceptor 51 and a developing member such as a sleeve 54 rotates in the direction of an arrow L with an axis of rotation 54a as a center to transport the developer to the photoreceptor 51. Developer-stirring and supplying means 55 rotates in the direction of an arrow L with an axis of rotation 55a as a center to stir the developer and simultaneously supply developing sleeve 54 with the developer. Developer-transporting means 56 rotates in the direction of arrow R with an axis of rotation 56a as a center to mixedly stir toner particles supplied from a toner-supplying portion (not shown) and excess or excessive developer returned along a partition plate 58 (to be discussed below) after regulation of the height of the developer supplied to sleeve 54, thereby supplying developer-stirring and supplying means 55 with the resulting mixture so as to be parallel to axis of rotation 51a of the photoreceptor 51.
Also, the case 52 is provided with a height-regulating member 57 for regulating the thickness or height of the developer on developing sleeve 54, formed in an upper portion within case 52. Partition plate 58 for returning excessive developer after regulating the height of the developer to developer-transporting means 56 is positioned above developer-stirring and supplying means 55. In this case, the partition plate 58 is formed in a chevron shape so as to not accumulate developer thereon. In addition, the case is provided with a toner-concentration sensor 59 in the vicinity of height-regulating member 57 so that a sensor 59a of toner-concentration sensor 59 faces an inclined surface 58a on an upstream side of the partition plate 58, with respect to the direction of returning movement of excess developer.
In a developing apparatus having the above described construction, the developing sleeve 54 is operated (e.g. rotated) to transport developer in the appointed direction during the time when copying is carried out to drive the developing apparatus. Thus, excessive developer after regulating the height of the developer is subjected to a centrifugal force resulting from such rotation to pass over the partition plate 58 and to be guided by the partition plate 58, as shown by arrows A in FIG. 16, and to be subjected to stirring in the appointed manner, and then the excessive developer is directed toward the developing sleeve 54, as shown by arrows B. Therefore, hardly any developer will be accumulated on the inclined upstream surface 58a of the partition plate 58 and or an inclined downstream surface 58b thereof.
However, upon completing a copying operation such that the developing apparatus will be in a non-driven condition, the operation of the developing sleeve 54 is stopped, so that excessive developer will be accumulated on both inclined surfaces 58a, 58b, as shown by reference numerals 60, 61. In particular, the accumulation of the developer on the inclined upstream surface 58a leads to the following disadvantages. That is to say, in a copying machine or the like wherein the developer comprises toner particles, which are insulating materials, and carrier particles, which are magnetic materials, and merely the toner particles are consumed, the toner concentration in the developer is reduced with the consumption of the toner particles. If this toner concentration reaches an appointed level or less, satisfactory developing cannot be achieved and thus a normal image cannot be obtained.
Thus, in a developing apparatus using the above described developer, in order to detect the toner concentration in the excessive developer, the toner-concentration sensor 59 is provided so as to face the inclined surface 58a on the upstream side, with respect to the developer returning direction, of the partition plate 58, thereby detecting such toner concentration. Accordingly, the toner concentration is prevented from reducing to less than the permissible value. Usually a coil is used as toner-concentration sensor 59 so that a change in magnetic permeability of the developer will be detected by a change in inductance of the coil.
However, if temperature and humidity within the case 52 become increased, the excessive developer accumulated on the inclined surface 58a will set due to such circumstances. Thus, the fluidity of the developer on inclined surface 58a is reduced, and it becomes difficult to detect a change in magnetic permeability of the developer by means of the toner-concentration sensor 59. As a result, it becomes difficult to exactly detect the toner concentration, and normal image formation is hindered.